Ignition timing control of a gasoline engine is conventionally performed, for example, in the following manner.
In particular, operating conditions of an engine are detected from a flow rate sensor for detecting an amount of intake air of the engine and an engine rotational speed sensor for detecting a rotational speed of the engine, and basic ignition timing information is determined, based on results of such detection from the sensors, from a two-dimensional map (ignition timing map) having lead angle values (ignition timing information) which depend upon an engine rotational speed Ne and a volumetric efficiency Ev (real volumetric efficiency Evr) obtained as a value A/N by dividing an intake air amount A by the engine rotational speed Ne, and then suitable correction is performed for such basic ignition timing information. Then, ignition means (ignition plugs, an ignition coil and so forth) is operated in accordance with the thus obtained ignition timing information to control the ignition timing of the engine.
By the way, since sampling of an intake air amount A of an engine generally delays upon acceleration of the engine, if an ignition timing is looked up from an ignition timing map in accordance with a real volumetric efficiency Ev (hereinafter referred to sometimes as Evr) based on the thus sampled intake air amount A, then an ignition timing on the partial side (lower intake air amount side) with respect to an ignition timing based on a real intake air amount will be looked up, and consequently, the thus looked up ignition timing exhibits a little lead, which will cause transient knocking.
Thus, in order to prevent such transient knocking, it is a possible solution to correct a volumetric efficiency (ignition timing map looking up volumetric efficiency) Ev (hereinafter referred to sometimes as Evm) for looking up an ignition timing from an ignition timing map so that it may have a value free from a delay thereby to indirectly retard the ignition timing.
For example, taking notice of a variation (deviation) .DELTA..theta. of a throttle opening as an index to an accelerating condition, the ignition timing map looking up volumetric efficiency Evm can be corrected in accordance with such throttle opening variation (throttle opening deviation) .DELTA..theta..
Curves (a) to (c) of FIG. 7 illustrate correction of such ignition timing map looking up volumetric efficiency Evm, and the curve (a) shows a throttle opening variation .DELTA..theta. upon acceleration of the engine; the curve (b) shows an ignition timing map looking up volumetric efficiency Evm corresponding to such throttle opening variation .DELTA..theta.; and the curve (c) shows a controlling condition of the ignition timing corresponding to such ignition timing map looking up volumetric efficiency Evm.
It is to be noted that the following description will proceed by way of an example of a four-cylinder engine.
When the throttle opening variation .DELTA..theta. increases after a point of time at which acceleration is judged as seen from the curve (a) of FIG. 7, the volumetric efficiency Evr will be corrected so that it may increase by such an amount as represented by a hatched portion of the curve (b) of FIG. 7 in response to a magnitude of the throttle opening variation .DELTA..theta..
Such correction is performed in accordance with the following expression (1): EQU Evm=Evr+(.DELTA..theta..times.G.sub.TH /4).times.K.sub.EVNE( 1)
where G.sub.TH is an ignition timing acceleration correction gain, and K.sub.EUNE is an ignition timing acceleration correction rotational speed coefficient.
It is to be noted that the ignition timing acceleration correction rotational speed coefficient K.sub.EUNE varies in such a manner as shown in FIG. 9 in response to an engine rotational speed Ne.
However, when the throttle opening variation .DELTA..theta. assumes its peak value, the peak value is held as a sampled value of the throttle opening variation .DELTA..theta. only for four strokes (for a period of four ignition timings) after then. Accordingly, for such period, also a corrected volumetric efficiency Evm is held at a value corresponding to such peak value.
The reason why a sampled value of the throttle opening variation .DELTA..theta. is held in this manner is that it is intended to compensate for an overshoot of a volumetric efficiency immediately after the throttle opening is put into a fully open condition.
Meanwhile, the ignition timing map looking up volumetric efficiency Evm and the ignition timing have, when the engine rotational speed Ne is constant, such a relationship as illustrated in FIG. 8, and generally, upon acceleration of the engine, the volumetric efficiency increases and the ignition timing presents a value on the delay side. Here, if the real volumetric efficiency Evr is corrected in accordance with the throttle opening variation .DELTA..theta. as seen from the curve (b) of FIG. 7 taking a delay of sampling of the real volumetric efficiency Evr into consideration, then the corrected ignition timing map looking up volumetric efficiency Evm is increased by such an amount as indicated by the hatched line portions comparing with the ignition timing map looking up volumetric efficiency Evm for which such correction has not been performed.
As a result, the ignition timing will be such as indicated by a broken line on the curve (c) of FIG. 7 and thus corrected to the delay side by such an amount as indicated by a hatched portion. Consequently, possible transition knocking is prevented.
By the way, if such correction of an ignition timing map looking up volumetric efficiency for determining a basic ignition timing as described above is performed, then a retard may possibly take place in the case of a specific acceleration pattern (in short, a specific throttle opening pattern) to cause incomplete acceleration.
For example, FIG. 10 is a time chart illustrating a correcting situation of a volumetric efficiency in the case of such specific acceleration pattern; and the curve (a) of FIG. 10 shows a throttle opening .theta. upon acceleration; the curve (b) shows a corresponding real volumetric efficiency (real Ev) Evr; the curve (c) shows a sampled value of a corresponding throttle opening variation .DELTA..theta.; the curve (d) shows a corresponding ignition timing map looking up volumetric efficiency Evm; the curve (e) shows a corresponding ignition timing controlling condition (retard correction amount); and the curve (f) shows a stroke of the engine.
In case the throttle opening .theta. varies in such a manner as shown by the curve (a) of FIG. 10, the throttle opening variation .DELTA..theta. will have two peak values h1 and h2 with such a small time difference as seen from the curve (c) of FIG. 10. And, it is assumed here that, when the throttle opening .theta. assumes the first peak value, the real volumetric efficiency (real Ev) Evr is in a saturated condition, and consequently, even if the throttle opening .theta. varies after then, the real volumetric efficiency Evr will not vary in value.
Meanwhile, the sampled value of the throttle opening variation .DELTA..theta. is held at a point of time when the throttle opening .theta. increases until it assumes the first peak value h1 as indicated at P1 (refer to the curve (c) of FIG. 10), and a value of a correction amount for a volumetric efficiency (refer to a hatched portion of the curve (d) of FIG. 10) then is held correspondingly. Then, the correction amount is, for example, added to the real volumetric efficiency Evr to determine an ignition timing map looking up volumetric efficiency Evm (refer to a hatched portion denoted by H1 of the curve (d) of FIG. 10). After such first time holding of a sampled value of the throttle opening variation .DELTA..theta., when a similar change takes place again by which the throttle opening .theta. is increased as denoted at P2, the sampled value of the throttle opening variation .DELTA..theta. which has been returned once to zero is held at a value of a suitable magnitude (refer to another portion h2 of the curve (c) of FIG. 10), and also a value of the volumetric efficiency correction value is held again (refer to another hatched portion H2 of the curve (d) of FIG. 10).
As a result, as seen from the curve (e) of FIG. 10, retard correction (refer to a reference character R2) will be repeated unnecessarily after transition knocking upon acceleration has been prevented (refer to another reference character R1). Accordingly, there is a problem that the output power of the engine is decreased by an amount corresponding to such retard amount and aimed acceleration of the engine will take place late as much.
The present invention contemplates solution to such problem as described above, and it is an object of the present invention to provide an ignition timing controlling system for an engine which can prevent knocking upon acceleration with certainty while minimizing a bad influence upon quick acceleration of the engine.